In recent years, the functionality of electronic equipment such as portable information devices and information home appliances has been further improved with the advancement of digital technology. The miniaturization and higher-speed operation of semiconductor elements for use in the electronic equipment are rapidly advancing with the improvement in the functionality of these electronic devices. Among these semiconductor elements, the applications of large-capacity nonvolatile memories represented by flash memories are rapidly expanding. Moreover, research and development are being advanced for variable resistance nonvolatile memory devices including variable resistance elements (ReRAMs) as the next-generation nonvolatile memories expected to replace such flash memories.
Here, the variable resistance element is an element having the characteristics that the resistance value (resistance state) is reversibly changed by applying an electric signal (voltage pulse) and the state is maintained. More specifically, the variable resistance element has a simple structure having a variable resistance layer formed between a first electrode layer and a second electrode layer using a variable resistance material. Information can be preserved in a nonvolatile manner by allocating a piece of the information to each resistance state of the variable resistance element. Specifically, for instance, two values can be preserved by allocating “0” to one of a low resistance state and a high resistance state, and allocating “1” to the other. Here, the low resistance state indicates a relatively low resistance value while the high resistance state indicates a higher resistance value than the low resistance state.
The resistance value of the variable resistance element is changed when the redox state of a variable resistance material is changed directly by electrical stimulation, i.e., through the gain or lose of electrons. This is different from a phase change memory (PCRAM) whose resistance value is changed when crystalline state is changed by heat generated by the electrical stimulation.
A nonvolatile memory device in which 1T1R memory cells are arrayed in a matrix is generally known as a nonvolatile memory device including variable resistance elements. Here, for example, the 1T1R memory cells each include a transistor and a variable resistance element connected in series at a cross point at which a bit line and a word line cross at right angles. Moreover, a crosspoint nonvolatile memory device including 1D1R memory cells arrayed in a matrix and a crosspoint nonvolatile memory device including 1D1R memory cells stacked in a multi-layer structure are known in an effort to achieve higher integration. Here, the memory cells each include a diode element and a variable resistance element connected in series at a cross point at which a bit line and a word line cross at right angles. In addition, the diode element serves as a current steering element for controlling the amount of current when a voltage is applied in the (LR) direction in which a resistance value is changed from a high resistance state to a low resistance state, in the normal operation.